The invention relates to a magnetic length or angle measuring system used, for example, for automatically determining positions, lengths or angles in mechanical engineering, automotive engineering or precision mechanics. The “absolute” system provides the corresponding values independent of the movement of the system components, in particular also at rest.
Such absolute magnetic length or angle measuring systems are known in the art. The measuring arrangement described in the published application DE 197 54 524 should be mentioned first. The scale is here implemented as a thread applied on a magnetic material, with the pitch individually changing from one winding to the next, in particular, with the pitch monotonously increasing in the measurement direction x. At least two magnetic field sensors which are spaced perpendicular to the measurement direction, are provided at the same x-value in the measurement head facing the scale. They are used to identify the corresponding thread pitch which can then be associated with the absolute position. Disadvantageously, the material of the scale must be soft-magnetic. Stray fields magnetize magnetic materials that can be used in practice, which may cause significant measurement errors. In addition, with variable pitch, very small values are required for the pitch. With such small values, however, the detectable changes in the magnetic field are particularly small, if the sensor and scale are not separated by a minimum distance. In addition, a necessary adjustment of auxiliary magnets for each sensor is complex.
The last disadvantage is eliminated with the arrangement according to DE 199 10 636 (FIG. 12, claims 16 and 17), because in this case a cylindrical hard-magnetic scale is used, wherein a magnetic thread with variable pitch is produced by magnetizing. The difference angle of the field directions between the positions of the two sensors can be determined with two magnetoresistive angle sensors which are spaced in the measurement direction. This difference can be absolutely associated with the position when using anisotropic magnetoresistive sensors, as long as the difference is less than 180°, with Hall sensors or GMR sensors up to 360°. The overall length of the absolute measurement is limited in that between the two sensors at the end of the measurement distance only one pitch more than at the beginning must exist. High-resolution is achieved by accurate evaluation of the angle value of one of the two sensors. As an additional disadvantage, manufacture of the magnetic threaded rod, in particular with a small pitch required to attain a high resolution of the length determination, is very complex. Rotation of the scale or parts thereof must be prevented, because it directly produces measurement errors for high-resolution.
An arrangement according to WO 2006/035055 no longer has the latter disadvantage. A cylindrical scale is here proposed which has in the measurement direction magnetic segments with alternating polarity and different length. Rotation of the scale does then no longer affect the measurement result. However, the invention according to WO 2006/035055 not only relates to cylindrical scales, but to all cross sections of scales where the segments of different length have alternating polarity. With two sensors spaced apart in the measurement direction which measure the angle of the direction of the magnetic field relative to the measurement direction, the phase difference between these two locations, which is proportional to the position of one of the two sensors, is deduced. For increasing the resolution of the position value, the number of the segment of the scale that faces the one sensor is first determined from the phase difference, with the resulting value of the angle of the field direction then indicating how far of the sensor has advanced relative to the start of the segment. The employed magnetic field sensors each consist of two sensor elements which are at the same location in the measurement direction and produce signals that depend on the field angle. It is assumed in the evaluation that the angle within each segment increases linearly with the position, or in other words, that the sensors provide values with respect to the respective segment length that are proportional to the sine and cosine of the field angle. The actual curve, however, deviates from the simplified assumptions for two reasons. On one hand, the magnetic field of the segments has harmonic contributions which decrease with increasing distance from the scale, with their amount determined by the segment length. Other hand, additional harmonic contributions are generated in the magnetoresistive sensors, which increase with decreasing field strength acting on the sensors. However, the field strength at the distance of the sensor varies strongly with the segment length so that continuously changing harmonic contributions are operating along the measurement direction. This limits the measurement accuracy, because the determined signal can be associated with a certain segment only over a limited number of segments, which diminishes the length resolution within the segment length.
DE 100 10 042 proposes linear magnetic measuring systems for application in the automotive industry. A large number of different configurations for the scale are disclosed. The problem with absolute measurements, however, is nowhere discussed in the document and can also not be inferred from the disclosed fact pattern. Accordingly, this document also does not provide any suggestion how absolute measuring systems for large measurement distances and a high-resolution can be designed.